Switching regulators are widely used in modern portable devices due to their high power conversion efficiency. Fixed-switching-frequency PWM voltage-mode control and current-mode control are the two main control schemes typically used. Compared to the current-mode control, voltage-mode control has the advantage of less sophisticated control circuitries, lower noise sensitivity, and potentially higher power efficiency due to the absence of accurate and fast current sensing and processing. Hence, it is a popular choice especially in single-output switching power supplies. Furthermore, in applications where large load current transient occurs and fast dynamic response is required, compensation methods with wide bandwidth are preferred, implying that Type II or Type III compensation is usually employed to extend the loop gain crossover frequency.
Another trend in power supply design in portable devices is the small form factor to fit in the limited physical space. Hence, small ceramic capacitors are gaining popularity to replace the bulky electrolytic capacitors as output filters of switching regulators. A ceramic capacitor has a much smaller equivalent series resistance (ESR) than its electrolytic counterparts. The resultant ESR-zero introduced in the loop gain is now pushed to higher frequency so that Type II compensation may not be enough to maintain adequate stability in the control loop. Type III compensation is hence the choice in many systems which demand high efficiency, fast bandwidth and compact form factor.
Nonetheless, the passive components of Type III compensator such as resistors and capacitors are often implemented off-chip. While this may give more design flexibility to some customers, it can be an obstacle to further squeezing the size of the power regulator, as there can be more than half a dozen of passive components required. On the other hand, integrating the passive components on-chip can be costly due to their large size required for generating zeros at low frequency. Moreover, the required high gain-bandwidth of the error amplifier driving the compensator is undesirable since it consumes considerable amount of power in the control circuitry.
Thus, there is a need for a compensation methodology that can generate the necessary zeros for compensation and yet has smaller area and power consumption requirements that allow for on-chip integration.